In recent years, as electronic devices have been reduced in size and increased in performance, increase in the density of printed wiring boards has been required. Since higher density printed wiring boards have a smaller conductive pattern, the conductive pattern tends to peel off easily from a base film. Therefore, as a substrate for a printed wiring board that satisfies the requirements for high density, a substrate for a printed wiring board on which a fine conductive pattern can be formed and that has high adhesion between a conductive layer and a base film is in demand.
To meet such requirements, a substrate for a printed wiring board has been proposed in which thin copper layers are formed on a heat-resistant base film having an insulating property without interposing an adhesive layer therebetween (see Japanese Unexamined Patent Application Publication No. 9-136378). The existing substrate for a printed wiring board includes the thin copper layers, each having a thickness in the range of 0.25 to 0.30 μm, which are formed on both sides of the heat-resistant base film having an insulating property by using a sputtering method; and thick copper layers, which are formed on the thin copper layers by using an electroplating method.